Lambda Determination Challenges for Ultra-Lean Hydrogen-Fueled Engines and the Impact on Engine Calibration

<div class="section abstract"><div class="htmlview paragraph">An increasing number of zero emission powertrain technologies will be required for meeting future CO₂ targets. While this demand will be met by battery and fuel cell electric vehicles in several markets, other solutions are needed for harder to electrify sectors. Hydrogen (H₂) internal combustion engines (ICEs) have become an attractive option for high power, high duty cycle vehicles and are expected to play a strong role in achieving zero emission goals. A unique characteristic of H₂ ICEs is their ability to operate extremely lean, with lambda (λ) greater than 2. At such conditions, a multitude of benefits are observed including higher thermal efficiency, lower engine-out nitrogen oxides (NO_x) emissions, and mitigating common problems with H₂ abnormal combustion such pre-ignition and knock. However, two critical issues arise during extreme enleanment of H₂ ICEs which have practical implications on controls and calibration of these engines. The first is the ability to properly measure air fuel ratio (AFR); both in a test cell environment and on-vehicle. The second is the deteriorating combustion efficiency with enleanment despite relative engine stability. In this study, several sources of error when measuring AFR for H₂ ICEs are discussed and quantified. A H₂-specific AFR equation is derived and the sensitivity to various measured combustion products is explored. It is shown that among these, H₂ fuel slip introduces the highest sensitivity to exhaust-measured AFR. The challenge this H₂ slip AFR sensitivity poses for closed-loop transient controls is explored and the impact on NO_x emissions is highlighted.</div></div>